GoogleTest: Restore support for empty discovered test list

Since commit 1cdceae8e3 (GoogleTest: Parse discovered test list from
JSON output if supported, 2025-05-02, v4.2.0-rc1~533^2~2) we've
incorrectly tried to `return()` from a `macro()` when no tests are
available.  This led to the situation that a file that contains the test
details was not properly overwritten but still indicated that the test
target was not built.

Fixes: #27319